This can happen for a few reasons, but the most common would be an "AVS Mismatch". This means that the billing address entered, does not match what your bank has on file for you.
If you check to make sure the billing information entered at checkout matches the address your bank has associated with your credit card, the order process should work smoothly :)
Please know that we also accept other forms of payment as well, such as Paypal or Check/ Money Order. If you are interested in paying using one of these methods instead of your credit card, let us know at email@example.com and we're happy to help you with that!